1. Field of the Invention
The present invention relates to methods for forming thick film patterns and to photosensitive pastes used for forming thick film patterns.
2. Description of the Related Art
Concomitant with the movement toward higher integration and higher processing of high-frequency electronic apparatuses, wiring conductors constituting high-frequency circuits provided in these electronic apparatuses are increasingly required to have a finer design, a larger thickness and a rectangular cross-section.
Heretofore, a thick conductor film forming a wiring conductor on a substrate is formed by the steps of preparing a conductive paste formed of an organic binder mixed with a powdered conductor, applying the conductive paste to the substrate by screen printing so as to form a desired pattern, and firing the desired pattern in order to remove the organic binder and to sinter the conductive component in the conductive paste.
However, the pattern mask accuracy used for screen printing is not always satisfactory, and for example, it has been difficult to form a fine pattern having a line width of about 100 xcexcm or less. Accordingly, proposed as a method for forming a fine pattern which cannot be obtained by screen printing, as disclosed in Japanese Unexamined Patent Application Publication Nos. 54-121967, 54-13591 and 59-143149, is a method of forming a fine pattern on a substrate by the steps of preparing a photosensitive paste formed of a photosensitive resin composition mixed with a powdered conductor, and applying the photosensitive paste to the substrate by a photolithographic technique so as to form the fine pattern on the substrate.
As the photosensitive resin composition for forming the photosensitive paste described above, a known photopolymerizable compound, a photo-modification compound or the like may be used, and for example, (1) a mixture of a monomer or an oligomer having reactive functional groups such as unsaturated groups and a photopolymerization initiator such as an aromatic carbonyl compound, (2) a so-called diazo resin such as a condensation product of an aromatic bis-azide and formaldehyde, (3) a mixture of an addition-polymerizable compound such as an epoxy compound and a photo-acid generator such as a diallyliodonium salt and (4) a naphthoquinone diazide compound may be used.
Among these materials described above, the mixture of the monomer having reactive functional groups such as unsaturated groups and the photopolymerization initiator such as an aromatic carbonyl compound (1) is particularly preferable.
When a conductor layer having a high electrical conductivity is formed, it is necessary to avoid defects such as wire breakage or crack generation which occurs while the conductor layer shrinks during firing, and as a result, it is necessary to increase the content of powdered conductor which is mixed with a photosensitive resin composition. However, when the content of the powdered conductor is increased, the optical transmittance of the photosensitive paste is decreased, resulting in insufficient curing in the paste, i.e., resulting in insufficient curing of the photosensitive resin composition contained in the paste. Accordingly, pattern formation becomes difficult to perform, and even when pattern formation is performed, a pattern having an inversed trapezoidial cross-section or having a so-called edge curl may be formed, significantly decreasing the transmission characteristics at a high frequency band.
The problems described above occur not only in a photosensitive paste containing a powdered conductor as a powdered inorganic material but also occur in a photosensitive paste containing a powdered insulator as a powdered inorganic material.
Accordingly, in order to solve the problems described above, an object of the present invention is to provide a method for forming a thick film pattern having superior high-frequency transmission characteristics, in which even when a photosensitive paste having a high content of a powdered conductor and thereby having a low optical transmittance is used, a desired pattern having a rectangular cross-section can be easily formed, and is to provide a photosensitive paste used for forming the thick film pattern.
Through intensive research by the inventor of the present invention in order to solve the problems described above, it was discovered that a thick film pattern having a rectangular cross-section and superior high-frequency transmission characteristics could be obtained by measuring the photocurable depth of a photosensitive paste film beforehand and subsequently determining the thickness thereof in consideration of the photocurable depth described above. After further experiments and studies were performed, the present invention was finally made.
The method for forming a thick film pattern of the present invention comprises the steps of measuring a photocurable depth d of a photosensitive paste beforehand; coating with the photosensitive paste in consideration of the photocurable depth d so as to form a photosensitive paste film having a predetermined thickness t; exposing the photosensitive paste film; and developing the exposed photosensitive paste film so as to form a thick film pattern having a predetermined shape.
Since the shape (the cross-section) of the thick film pattern to be formed can be estimated to some extent from the photocurable depth d of the photosensitive paste and the film thickness t of the photosensitive paste film, a thick film pattern having a desired cross-section can be efficiently formed by measuring the photocurable depth d of the photosensitive paste beforehand and by determining the thickness t of the photosensitive paste film in consideration of the photocurable depth d. Consequently, a thick film pattern having superior adhesiveness to an object (typically, a substrate) on which the thick film pattern is formed and having a high shape accuracy can be formed.
When the thickness of the photosensitive paste film is larger than the photocurable depth to some degree, the thickness pattern to be formed tends to have a cross-section in an inversed trapezoidial shape to some extent; when the thick film pattern to be formed is not required to have a high shape accuracy, a thickness larger than the photocurable depth to some degree may be employed as the thickness of the photosensitive paste film.
In the present invention, the photosensitive paste means a paste containing a powdered inorganic material and a photosensitive resin component, and when necessary, the photosensitive paste may contain a storage stabilizer such as a solvent or a polymerization inhibitor, an anti-oxidant, a dye, a pigment, a defoaming agent, a surfactant, or the like. The type and the composition of the photosensitive paste are not specifically limited, and as long as the advantages of the present invention can be fully obtained, photosensitive pastes having various compositions may be used.
In the present invention, the photosensitive paste film means a film in a dry state which is formed by applying the photosensitive paste to a base body, such as a ceramic substrate or a PET film, by a known technique, such as screen printing or spin coating, and by drying the photosensitive paste on the base body. The thickness of the photosensitive paste film means the thickness of the film in a dry state.
In addition, the photocurable depth d of the photosensitive paste film means the depth from the surface of the film toward the inside thereof at which the energy of radiated light such as ultraviolet light is reduced to less than the minimum energy required for curing the resin component of the film either by being consumed by curing the resin component or being attenuated due to the absorption therein while the light is passing through the paste film.
As a method for evaluating (measuring) the photocurable depth described above, the following method may be mentioned by way of example.
(1) First, the photosensitive paste film is formed on a substrate such as a glass by screen printing.
(2) Next, exposure is performed on the photosensitive paste film by applying a predetermined amount of energy of light (such as ultraviolet light).
(3) Subsequently, development is performed for the exposed photosensitive paste film by using an aqueous solution containing sodium carbonate or the like at a small concentration so as to remove noncured portions, and the thickness of the remaining cured film is then measured and is determined as the photocurable depth d.
In the measurement described above, the thickness can be measured by an optical microscope, an electron microscope, a laser microscope, or the like.
In addition, the coating with the photosensitive paste in the method for forming the thick film pattern of the present invention is preferably performed so that the relationship between the photocurable depth d of the photosensitive paste and the thickness t of the photosensitive paste film satisfies the equation txe2x89xa6d.
When the coating with the photosensitive paste is performed so that the equation, txe2x89xa6d, is satisfied, that is, when the thickness t of the photosensitive paste film is equivalent to or less than the photocurable depth d, the entire photosensitive paste film in the thickness direction thereof can be reliably cured so as not to be dissolved during development. Accordingly, a thick film pattern having a desired cross-section can be reliably formed, and hence, a thick film pattern having superior adhesiveness to an object (typically, a substrate) on which the thick film pattern is formed and having a rectangular cross-section can be efficiently formed.
In addition, when this thick film pattern is fired, the rectangular cross-section thereof is retained, and edge curl, which occurs when a thick film pattern has an inversed trapezoidial cross-section, does not occur. Consequently, formation of electrode surface pointed portions at which a current is concentrated is avoided, and hence, the current concentration can be suppressed, whereby improved transmission characteristics at a high frequency can be obtained.
When the thickness of the photosensitive paste film is larger than the photocurable depth, the degree of undercutting formed during development becomes significant, and as a result, the cross-section of the photosensitive paste film tends to have a cross-section in an inversed trapezoidial shape. When a thick film pattern to be formed does not require a high shape accuracy, the film thickness of the photosensitive paste film may be determined to be larger than the photocurable depth to some degree. However, when a thick film pattern having a rectangular cross-section and having a high shape accuracy is formed, the thickness t of the photosensitive paste film must be controlled so as not to exceed the photocurable depth d.
In the method for forming the thick film pattern according to the present invention, the photosensitive paste may comprise a powdered inorganic material having an average particle diameter m, and the coating with the photosensitive paste is preferably performed so that the thickness t of the photosensitive paste film satisfies the equation, txe2x89xa6d+m, in which d is the photocurable depth of the photosensitive paste.
When the content of the powdered inorganic material is increased, the probability of the presence of the powdered inorganic material on the surface of the photosensitive paste film is increased; however, when the thickness t of the photosensitive paste film satisfies the equation, txe2x89xa6d+m, in which m indicates the average particle diameter of the powdered inorganic material contained in the photosensitive paste film, the entire photosensitive paste film in the thickness direction thereof can be substantially cured. As a result, a thick film pattern having a desired cross-section can be reliably formed, and hence, a thick film pattern having superior adhesiveness to an object (typically, a substrate) on which the thick film pattern is formed and having a high shape accuracy can be efficiently formed.
According to the method for forming the thick film pattern described above, for example, even when the content of the powdered inorganic material is high, and hence, when the optical transmission of the photosensitive paste film is low, a thick film pattern can be reliably formed, and a pattern having a rectangular cross-section and superior high-frequency transmission characteristics can be efficiently obtained.
When at least two types of powdered inorganic materials are used, the average particle diameter m of the powdered inorganic material described above means the average particle diameter of a powdered inorganic material having the highest content compared to those of all other inorganic materials.
In addition, a thick film pattern in the method for forming the thick film pattern of the present invention having an approximately rectangular cross-section in the direction perpendicular to the longitudinal direction of the thick film pattern is preferably formed.
The method for forming the thick film pattern of the present invention is a method suitably used for forming a thick film pattern having a high shape accuracy, and hence, a thick film pattern having a rectangular cross-section can be reliably formed. Hence, a thick film pattern having superior adhesiveness to a substrate or the like, superior high-frequency transmission characteristics and a rectangular cross-section can be efficiently formed.
A photosensitive paste according to the present invention is used for forming a thick film pattern having an approximately rectangular cross-section in the direction perpendicular to the longitudinal direction of the thick film pattern, and comprises a powdered inorganic material having an average particle diameter m; wherein the average particle diameter m satisfies the equation, txe2x89xa6d+m, in which d indicates the photocurable depth of the photosensitive paste, and t is the thickness of a film formed of the photosensitive paste. The photosensitive paste used satisfies this equation, and accordingly, since the entire photosensitive paste film formed by coating is reliably cured in the thickness direction thereof, a thick film pattern having a rectangular cross-section can be reliably formed.
Consequently, a thick film pattern having superior adhesiveness to an object (typically, a substrate) on which the thick film pattern is formed, superior high-frequency transmission characteristics, and a rectangular cross-section can be efficiently formed by using the photosensitive paste of the present invention, and hence, the advantages of the present invention can be fully realized.